1. Field of the Invention
The present invention relates to a process and apparatus for producing a polyurethane mixture for use in manufacturing structural panels or molded pieces such as, for example, door panels for automobiles or other vehicles and the like.
2. Description of the Related Art
Inert filler material, such as reinforcing fiber, is often used in the production of structural panels or, more generally, molded pieces of reinforced polyurethane material. Inert filler material improves both the rigidity and the mechanical properties of molded pieces of reinforced polyurethane material and structural panels.
For the purposes of the present invention, the term xe2x80x9cfillerxe2x80x9d is understood to mean any solid, inert and fluidizable material, including also any material which can be transported by an air or gas flow. Such filler material can be used for filling and reinforcing purposes and can be suitably pretreated waste material. The filler material may therefore be in the form of powder, granulates, fibers of various lengths, continuous fibers, or any other suitable material which may be fluidized by an air flow, such as natural or synthetic fibers, a finely ground synthetic material, or recycled foams, finely chopped wood, milled cork, sawdust or the like.
For the purposes of the present invention, the term xe2x80x9clong fibersxe2x80x9d is understood to mean fibers whose length is equivalent to or greater than 3-5 mm, up to several centimeters or more, not excluding the use of continuous fibers.
When fillers are used in molded products, there is often a problem in obtaining an adequate mixture of the chemical reactants with the filler material, especially where the filler material is long or fragile. GB-A-1,245,216 and U.S. Pat. No. 4,397,407 both discuss methods of embedding a fluidizable filler material into formable mixtures of reactive chemical components.
In GB-A-1,245,216 filler material of reinforcing fibers is fed directly into a mixing chamber. The reactive chemical components are injected into the mixing chamber at right angles to the reinforcing fibers as they flow through the mixing chamber.
This process is unsatisfactory particularly where the filler material comprises long or very long fibers, fed in large amounts. The air flow transporting the filler, and the filler material itself, prevent close contact and homogeneous mixing of the reactive chemical components with the filler. Consequently, the resulting molded product contains defects which makes it commercially unacceptable.
U.S. Pat. No. 4,397,407, in contrast, describes a process where a foamable mixture of reactive chemical components is first prepared in a separate high-pressure mixing chamber and then fed by a long duct into an annular chamber where the reactive chemical mixture is blended with a filler, such as a granular material or short reinforcing fibers. The feeding duct leads to one side of the annular chamber, opposite to a discharge duct, and is formed by a movable tubular element which is connected to the mixing chamber by an additional channel.
This procedure also leads to inadequate mixing of the fillers with the reactants. In fact, when the reactive liquid mixture comes into contact with the flow of fibers or filler material, the reactive liquid mixture has lost most of its turbulence and kinetic energy inside the long feeding duct. By the time the reactants and the filler are combined, they have both assumed a substantially parallel flow.
The mixing procedure described above is also unsatisfactory where the filler is composed of a delicate material, such as glass fibers, which require simple, linear paths without pronounced curves and deviations.
U.S. Pat. No. 4,332,335, the complete disclosure of which is incorporated herein by reference, describes a head for mixing and ejecting reactive liquid components. The device described therein features a small mixing chamber separate from the discharge duct in which mixing takes places under high turbulence condition so as to prevent problems arising from imperfect mixing of the reactive liquid components. In the use of this mixing head the flow of the mixture is directly injected into a closed cavity of a mold or poured into an open mold by reciprocating the mixing head.
GB-A-1,579,543 and WO-A-96/35562 describe other apparatus where a tubular flow of liquid polyurethane material passes through an annular duct and comes into contact outside of that duct with reinforcing fibers axially flowing through a feeding channel concentrically arranged to the annular duct.
In particular, WO-A-96/35562 (xe2x80x9cWO""562xe2x80x9d) describes a device for the production of molded pieces of formable synthetic material containing long reinforcing fibers and suggests that a high-pressure mixing head can be used for the production of polyurethane materials. In WO""562 the polyurethane mixing chamber is connected to an annular discharge duct via an intermediate duct, so that the reactive polyurethane mixture is separately prepared and fed into the discharge duct in the form of a tubular flow. The tubular flow of reactive polyurethane mixture then comes into contact with the fiber reinforcing material.
According to the examples of FIGS. 1 and 2 of WO""562, filler reinforcing fibers are fed into the discharge duct through a separate pipe member which coaxially extends along and beyond a cleaning member reciprocating inside the annular duct to discharge the mixture.
The use of a movable separate cleaning member to feed the reinforcing fibers may severely limit the performance of this type of apparatus, since the tubular cleaning member comprises inner and outer surfaces adhering to the cylindrical surfaces of the annular discharge duct. These surfaces are completely wetted by the polyurethane mixture during each supply step. The film of the mixture remaining between the interfaces thus tends to cause the cleaning member to adhere strongly to the double surface of the annular duct thereby completely blocking movement of that cleaning member. To avoid this blocking problem, a large hydraulic cylinder must be used for the cleaning member. This hydraulic cylinder must be able to exert a strong enough force to release the cleaning member during its rearward movement. The necessity of using a hydraulic cylinder with these capabilities increases the weight of the entire mixing apparatus and makes reciprocation of the mixing apparatus over the cavity of a mold more difficult.
Furthermore, given the fact that the adhesion forces between the contacting surfaces of the discharge duct and the cleaning member, with a length equal to or slightly more than five times its diameter, exceed the tensile stress of steel, it must be concluded that the dimensions of the resistant cross-sections of the cleaning member must be largely increased in order to prevent the tubular cleaning member and the fiber feeding pipe member from breaking or becoming damaged.
Finally, the use in WO""562 of an intermediate connecting duct between the mixing chamber and the annular duct discharging the polyurethane mixture dampens the flow of the polyurethane mixture thereby causing the reinforcing fibers to not be effectively mixed and homogeneously wetted with the reactant mixture.
Furthermore, the intermediate connecting duct will require its own cleaning member and corresponding hydraulic operating cylinder.
All these factors lead to complications in designing mixing apparatus and considerably increase the weight and dimensions of the apparatus itself.
Although WO""562 suggests that the design of the apparatus could be simplified by connecting the mixing chamber directly to a discharge duct, into which the pipe member for feeding the reinforcing fibers extends, this design would still suffer from the drawbacks discussed above.
WO""562, therefore, fails to disclose a suitable, lightweight apparatus design which would allow for improved wetting of fillers and operating conditions. The general teaching of WO""562 is limited to providing a separate, centrally located pipe for feeding fibers, which extends axially through and beyond the longitudinal bore of the cleaning member, to protrude and extend throughout the annular discharge duct so as to form the polyurethane mixture into a tubular flow surrounding the central flow of the reinforcing fibers with both tubular and central flows moving coaxially in the same direction.
In the general production of molded pieces of polyurethane material, particularly in molding pieces, containing a fluidizable filler, it is of great importance to have apparatus or mixing devices which are of simple design and extremely lightweight, yet allow for a high degree of mixing and impregnation of the filler material with the liquid polyurethane mixture before the polyurethane mixture and the filler material reach the inside cavity of a mold or are poured onto the surface of an underlying substrate.
Weight is an essential factor in the use of such mixing apparatus since it is normally manipulated over the substrate surface by the reciprocating arm of a positioning device, such as a robotic device. The mixing apparatus has to be moved quickly and reciprocated in various directions, along a predetermined path, subjecting the apparatus to sudden changes in direction and to intense braking and accelerating forces at each reversal of movement, so as to prevent undesirable accumulations of material from occurring locally.
Furthermore, in the production of large-sized pieces, for example automotive door panels, it is in general necessary to carry out a considerable number of strokes, for example 10 or more, by the mixing apparatus in order to cover the entire surface of the mold. Such activity also frequently occurs within an extremely short period of time, for example 8-10 seconds, in order to complete the discharge before the polyurethane mixture begins to react.
In the general use of mixing apparatus for adding fillers to polyurethane material, as described in DE-A-19618393 and DE-U-29704560, the flow of the resulting polyurethane-filler material mixture, which must be distributed in the cavity of a mold or on the surface of an underlying substrate, tends to assume a substantially conical or flat shape. The dimensions of this flow depend upon the design of the mixing apparatus and the mixing process, as well as the methods by which the two flows are combined or mixed with one another. Accordingly, the width of surface coverage of each stroke of the mixing apparatus in a given direction on the underlying substrate depends not only on the distance between the same mixing apparatus and the underlying substrate, but also to a large extent on the characteristics of the impregnated filler and/or reinforcing material, and on the methods by which the polyurethane mixture and the filler are combined or blended.
Generally, the width of surface coverage of each stroke cannot be controlled in any way. Therefore, when large panels or pieces with large dimensions are produced, the mixing apparatus must carry out several strokes, reciprocating across the underlying substrate. The mixing apparatus must cover the underlying substrate within an extremely short period of time before the polyurethane material begins to react.
Due to the considerable weight of the mixing apparatus as well as the forces of inertia exerted at each change or reversal of movement, extremely sturdy and bulky operating systems are required. Moreover, at high speeds at which the mixing apparatus is moving, the distribution of the mixed material may, in some cases, be non-uniform, creating an unsatisfactory product. Finally, a polyurethane-filler mixture, especially if the filler is composed of long fibers, has little or no tendency to flow on the deposition surface. Thus it tends to remain in the zone where it was initially deposited.
It is, therefore, essential to have a mixing apparatus which is not only lightweight and simple in design, but, at the same time, able to supply large quantities of polyurethane mixture within extremely short periods of time. In the case a filler material is blended with the polyurethane mixture, it is also important that the filler material, especially if composed of long fibers, be impregnated and wetted substantially homogeneously by the polyurethane mixture directly inside the same mixing device in order to ensure a good impregnation and a substantially homogeneous distribution within a short time. These characteristics are needed to achieve optimum structural properties in the resulting manufactured panels or molded pieces.
It is, therefore, an object of the present invention to provide a high-pressure, self-cleaning mixing apparatus for the production of molded pieces of polyurethane material containing a filler. This mixing apparatus responds to the need for a simple design and an extremely lightweight apparatus. This design also allows for rapid displacements without generating great resistance to accelerations at each reversal or change of its movement.
Another object of the present invention is to provide a mixing apparatus which improves the impregnation of filler material inside the mixing apparatus. This impregnation takes place under high turbulence conditions so that, in the molded pieces, the filler becomes strongly bonded to the polyurethane material of the molded composite pieces.
Yet another object of the present invention is to provide a self-cleaning, high-pressure mixing apparatus for practical, universal use. Fluidized filler material of any type or nature can be additionally fed into the apparatus by a flow of entraining air.
The apparatus according to the present invention can therefore be used for a large number of applications. For example, it can be used in the molding a wide variety of mechanical or structural pieces. It can also be used in expanded foams, both of rigid and flexible types, in which the filler may advantageously be a new, reused or recycled material, thus allowing for the production of more economical and relatively low-cost molded pieces.
Another object of the present invention is to provide a process and apparatus for the production of polyurethane material, and to obtain the deposition of the mixture on a larger area of a substrate or the cavity of a mold in a short time. The process of the present invention can be carried out within a relatively short period of time maintaining a homogeneous distribution of the mixture throughout the deposition. All of this accomplished without cumbersome and complicated operating systems for moving the mixing apparatus itself.
Another object of the present invention is to provide a process and apparatus as defined above, which allow the mixing apparatus to move at a relatively low speed, thus reducing the problems associated with the inertia phenomenon and reversal or variations in the movement of the apparatus.
Yet another object of the invention is to provide a process and apparatus for the production of polyurethane material, as defined above, which in addition to allowing for a uniform distribution of the sprayed or deposited material, also produces a product having good characteristics and allows for an adequate control of the spraying or deposition of the polyurethane and/or entrained reinforcing material discharged from the apparatus, for example, into a mold.
A further object of the invention is to provide both a process and apparatus, as defined above, which allow for control over the deposition of the polyurethane and/or entrained reinforcing material depending on the dimensional characteristics, the shape of the mold or of the molded piece and the relative movement required between the apparatus and the mold of surface on which the deposition is occurring.
A first innovative aspect of the invention lies in the simultaneous injection or co-injection of polyurethane mixture and a filler of non-reactive material which is achieved using a mixing apparatus of special design in which the meeting point between the polyurethane mixture and the filler may be kept close to the mixing chamber to improve wetting or saturation of the filler before the blend reaches the surface of a mold.
A second innovative aspect of the invention lies in the use of air jets to deviate the flow of the polyurethane mixture or the flow of the blended mixture of polyurethane and filler material to obtain and control the deposition of the mixture on a large area of a substrate or into the cavity of a mold, in a short time by a reciprocating movement in the mixing device.
Other objects, features, and characteristics of the invention will become apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification and wherein reference numerals represent corresponding parts in the various views.